Heretofore, in producing cellulose acylate films for use in liquid-crystal image display devices, a solution-casting method has been principally carried out, which comprises dissolving cellulose acylate in a chlorine-containing organic solvent such as dichloromethane, casting it on a substrate, and drying it to form a film. Dichloromethane has heretofore been used as a good solvent for cellulose acylate, and since its boiling point is low (about 40° C.), it is favorably used owing to its advantage that it may be readily vaporized in the film-forming and drying step in its production process.
Recently, from the viewpoint of environmental protection, it has become required to significantly retard release of a chlorine-containing organic solvent having a low boiling point in a step of handling it in closed facilities. Accordingly, for example, employed is a method of preventing solvent leakage through a thorough closed system, and even if an organic solvent leaks out by any chance, employed is a method of installing a gas absorption tower to adsorb and treat it before it is released in outdoor air. Further, before discharged, a chlorine-containing organic solvent is burnt with flames or is decomposed with electron beams, whereby the organic solvent is not almost discharged out. However, it is still impossible to completely prevent the release of organic solvent, and further studies are required.
A melt-casting method of producing a film of a specific cellulose acylate has been proposed as a film formation method not using an organic method (Patent Document 1). According to the method, the carbon chain of the ester group in cellulose acylate is prolonged so as to lower the melting point of the polymer for easy melt-casting film formation of the polymer. Concretely, cellulose acetate is changed into cellulose propionate or cellulose butyrate, thereby enabling melt-casting film formation of the polymer. We, the present inventors tried forming a polarizer, using a film produced according to the melt-casting film formation method described in this patent document, and tried building the polarizer in a liquid-crystal display device, but we knew formation of blurry images. Accordingly, the improvement was desired.    Patent Document 1: JP-A-2000-352620